The development of cisplatin resistance by the cancer cell is one of the major problems in treating patients with cisplatin chemotherapy. The defective accumulation of cisplatin has emerged as a salient feature of cisplatin resistance. Unfortunately, there is little information about the mechanisms of cisplatin resistance in breast cancer. The objective of this investigation is to determine the relationship between a terbium binding protein and the accumulation of cisplatin in cisplatin-sensitive and cisplatin-resistant human breast cancer cells. More specifically, the proposed research will determine whether terbium affects the accumulation and cytotoxicity of cisplatin in breast cancer cells. The proposed research also intends to determine whether the number of terbium binding sites and/or the terbium receptor affinity of cisplatin correlates with 1) the cytotoxicity of cisplatin and/or 2) the accumulation of cisplatin in sensitive and resistant breast cancer cell pairs. Time-resolved intensities of terbium phosphorescence will be used to obtain equilibrium binding constants for the receptor binding of cisplatin and terbium in dissociated cell suspensions. The amount of cisplatin taken up by the cells in culture will be measured via atomic absorption spectrophotometry. The cytotoxicity of cisplatin will be determined by the monolayer colony assay. The hypothesis of this proposal is that the terbium receptor plays a central role in the accumulation of cisplatin in breast cancer cells. If our results are encouraging, we will attempt to predict the development of cisplatin resistance in fresh human tumor specimens, based on the receptor binding of cisplatin and terbium.